The painting is “Knowledge and Wonder,” a dreamlike frieze that the artist completed in 1995 for the Legler branch of the Chicago Public Library — on the city’s poorer West Side, where African-Americans make up about 44 percent of the population. This week, Rahm Emanuel announced that the library would sell the painting at Christie’s with the proceeds — the estimate is $10 million to $15 million — earmarked to expand library services to the same level as other major branches.